Engine rattle

[Burns]

Are you sure that it is the motor that is rattling?  It could be almost anything that is loose. As suggested check the torque on your header bolts.  If that's not the problem put a screwdriver handle in your ear and use the business end to locate/eliminate the source of the noise.  Since there is no noise at idle I doubt that it is a motor problem, but I'd start with the head anyway.

Good luck

[Pat Conlon]

Ron, after ruling out loose bolts, did you see this? http://www.fjowners.com/index.php?topic=11667.0

If the cam caps are tight, pull out the cam chain tensioner and check it, and reinstall it.....
http://www.fjowners.com/index.php?topic=5620.0

[breeze48552000]

Exhaust header pipe ? 
George

Single wall pipes on 86/87...

it could possibly be engine overheating a bit after its ran causing valves to chatter as they loosen up from heat?

[FJ_Hooligan]

As the engine heats up, valve lash is diminished due to thermal expansion of the cams and the valves themselves. 

That's why you measure/adjust the valve clearances with a COLD motor.